1. Field of the Invention
The present invention relates to an ultrasound observation system that generates an image including an elastic image based on an ultrasound signal obtained by transmitting and receiving ultrasound.
2. Description of the Related Art
An ultrasound diagnostic apparatus that generates an elastic image indicative of hardness or softness of a living tissue from a distortion amount of the living tissue caused by an oppression force has been put into practical use.
For example, in Japanese Patent Laid-Open Publication No. 2004-351062, there is described an ultrasound diagnostic apparatus in which external forces are applied to a subject by a surgeon using a probe and a plurality of tomographic image data which are different in time are acquired, and not only displacement of respective tissues but also modulus of elasticity are calculated based on the acquired tomographic image data, to create an elastic image. Further, there is described a technique of measuring a pressure in order to obtain an appropriate elastic image and displaying an appropriate pressure range.
Further, in International Patent Publication WO2006/041050, there is described an extracorporeal ultrasound probe capable of performing elasticity measurement which is configured to be able to perform automatic pressurization so as to obtain an appropriate elastic data. Further, it is described that an oppressing speed of an oppressing action is controlled in the automatic pressurization based on period information of an ultrasound reception signal frame data.
Incidentally, in a case where the subject is a living body or the like, there is a case where a pressure action having a certain rhythm due to spontaneous displacement of the subject such as a pulse beat or a pulsation is found. Therefore, it is conceivable to obtain an elastic image based on such spontaneous displacement of the subject.